Field emission devices are known in the art. Methods for fabricating cone-shaped electron emitters, including Spindt-tip emitters, are also known in the art.
In one prior art scheme for fabricating cone-shaped electron emitters, a combination of a substantially normal vapor deposition process and a low angle vapor deposition process are employed. It is known in the art to form an array of field emitters by forming a plurality of vias (emitter wells) in a dielectric layer and then depositing the emitter material so that one emitter cone is formed in each via. Each emitter well opening typically has a diameter in the micron range. The low angle vapor deposition provides material which continually reduces the size of the opening of the via, thereby continually reducing the diameter of the deposited material within the via. The material forming the cone is provided by the substantially normal vapor deposition process.
Another prior art scheme for forming cone-shaped field emitters includes evaporative deposition, such as by boiling or electron-beam evaporation of a field emissive material, such as molybdenum. Evaporation of tips is typically performed in a high vacuum, at pressures less than or equal to about 1.times.10.sup.-7 Torr. This process is inherently collimated because the molecules depart generally radially from the source and because, subsequent their departure, they are generally not deflected by other molecules. However, the spray of molecules comprises a cone wherein the species nearer the circumference are deposited at an angle. In this manner, the deposition over the substrate varies from a substantially normal deposition at the center of the spray cone, to an angled deposition at the circumference of the spray cone. The angularity of the deposition may be tolerated to about an 8.degree. half angle of the apex of the spray cone. For half angles greater than 8.degree., the cones formed at the outer portions of the deposition substrate are no longer sufficiently centered within the vias. To achieve the necessary control and uniformity of emission over the substrate, the cones must all be substantially centered within the vias.
Another disadvantage of this prior art evaporation process is that, as substrate size increases, the distance between the target and substrate must be increased to maintain the same maximum deposition angle. The increased separation between substrate and source requires an increase in volume of the deposition tool. This translates to greater maintenance requirements and a more involved evacuation process. For substrates having diameters greater than about 16 cm, the distance between substrate and evaporation source must be greater than about 60 cm. In general, this distance scales linearly with respect to substrate size.
Also known in the art is the use of a collimator which has a collimation cell diameter on the order of the dimension of a pixel, which is about a couple hundred micrometers. In this prior art scheme, the collimator is static and physically rests on the substrate surface. This configuration is completely inadequate for production scale operations because it requires tedious alignment. It also results in a variation of tip shapes and sizes over each pixel area.
Accordingly, there exists a need for an improved method for fabricating an array of conical electron emitters which is low-cost, simple to perform, efficient, and provides uniform geometry of conical emitters in large-area substrates.